1. Field of the Invention
The present invention relates generally to semiconductor devices and related methods of manufacture. More particularly, the invention relates to semiconductor devices having a dual metal gate structure and related methods of manufacture.
2. Description of the Related Art
Constant demand for evermore densely integrated semiconductor devices, including the demand for semiconductor memory devices of constantly increasing capacity, has resulted in relentless pressure to reduce the size of the constituent components forming contemporary semiconductor devices. For example, the physical dimensions of nearly every constituent component in conventional Complementary Metal Oxide Semiconductor (CMOS) devices have been dramatically reduced over the past several years. This is particularly true of CMOS memory devices. However, despite the constant reduction in the physical dimensions of their constituent components, contemporary CMOS memory devices must nonetheless meet increasingly demanding performance criteria.
This ubiquitous “scaling down” of CMOS memory devices necessitates the use of ever more thin, yet performance criteria-compliant gate insulating (e.g., dielectric) layers. As the design rule for CMOS memory devices headed below 100 nm during the past decade, it has become increasingly apparent that the polysilicon gate structures conventionally used would no longer work with very thin gate insulating layers.
The use of polysilicon gate electrodes is a time honored one. Polysilicon is easy to deposit and pattern. It is not adversely affected by subsequently applied high temperature processes, and its “work function” is easily modified by selectively doping the polysilicon.
All conductive materials, including semiconductors, are characterized by a certain responsiveness to applied energy. This responsiveness is termed the material's “work function,” and is usually expressed in electron volts (eV). This intrinsic quality of the material is defined by the minimum quantity of energy required to remove an electron from the Fermi level of the material in a vacuum. Different materials have different Fermi levels, different electron configurations, and thus require different amounts of applied energy to remove an electron.
In all materials, electrons arrange themselves in a hierarchy of energy states, first filling up the lower energy states before filling the higher energy states. The Fermi level for a particular material is associated with a highest occupied energy state for the material at zero temperature.
The Fermi level, and therefore the corresponding work function, for many undoped semiconductor materials like polysilicon is situated somewhere generally mid-way between the so-called conductance band of silicon (about 4.1 eV) and the valance band of silicon (about 5.2 eV). (This type of work function is referred to hereafter as “mid-bandgap.”)
In contrast, conventional semiconductor materials have been selectively doped to produce either an N-type or a P-type material. N-type semiconductor materials have a Fermi level closer to the conduction band of silicon than to the valance band of silicon. P-type semiconductor materials have the opposite characteristic.
PMOS and NMOS type devices, such as transistors, are formed in great number in contemporary semiconductor devices. Each of these device types operationally benefits from a gate electrode having, respectively, P-type and N-type performance characteristics, including P-type and N-type work functions. Accordingly, conventional polysilicon CMOS gate electrodes are routinely doped with selected P-type and N-type impurities in order to modify (“adjust”) the mid-bandgap work function of undoped polysilicon to levels more appropriately suited to (i.e., “compatible” with) PMOS and NMOS devices, respectively.
Unfortunately, when used with very thin gate insulating layers doped polysilicon gate electrodes experience undesirable voltage drops and unacceptable drive current requirements due to a well understood phenomenon called the gate depletion effect. Doped polysilicon gate electrodes may also experience high gate resistance, boron (B) penetration problems, and stability issues related to high-k gate dielectrics.
Beginning in the late 1990's, researchers proposed the use of metal gate electrodes as a replacement for the polysilicon gate electrodes previously used in CMOS devices. Various metals were identified as possible candidates for the formation of metal gate electrodes, including tungsten (W), titanium (Ti), tantalum (Ta), molybdenum (Mo), ruthenium (Ru), nickel (Ni), and niobium (Nb). Various metal compounds, metal nitrides, metal silicides, and metal oxides were also proposed for use in the formation of metal gate electrodes. (Hereafter, metal compounds, alloys, nitrides, silicides, and oxides are all subsumed in the term “metal” as it relates to the selection and formation of metal layer adapted for use in the fabrication of gate electrodes).
While certain types of metal gate electrodes experience some chemical and thermal stability issues, as compared with polysilicon gate electrodes, metal gate electrodes generally enjoy low gate resistance and do not suffer from the gate deletion effect. However, metal gate electrodes are significantly more difficult and complicated to fabricate. This is particularly true of etching process(es) used to pattern the metal layer. Such process(es) use chemicals that are difficult or dangerous to handle, use, and dispose of after use.
Given these enhanced processing difficulties, it is generally preferred to form only a single metal layer having a mid-bandgap work function from which PMOS and NMOS metal gate electrodes are subsequently formed. That is, manufacturing methods requiring multiple metal layer depositions and multiple corresponding etching steps are disfavored. However, it must be noted that some manufacturers have adopted a multiple metal layer deposition approach to the problem of providing PMOS and NMOS-compatible gate electrodes, despite the increased processing difficulties. Some companies feel that the ability to precisely select disparate metals for the formation of PMOS and NOMS gate electrodes allows maximum flexibility in the definition of work function and the selection of an operating threshold voltage (Vth) for the device incorporating the PMOS or NMOS gate electrode.
U.S. Pat. No. 6,130,123 to Liang et al. recognizes the utility of forming dual metal gate electrodes (i.e., PMOS and NMOS metal gate electrodes) from a single metal layer. Selected portions of the single metal layer are adjusted for PMOS and NMOS operation by adjusting their respective Fermi levels. Liang et al. propose in one example a method wherein selected portions of an N-type metal layer having a work function suitable for NMOS operation is exposed to a nitrogen-rich (NH3 or N2) ambient environment in order to change the work function of the exposed selected portions into a work function suitable for PMOS operation.
U.S. Pat. No. 6,483,151 to Wakabayashi et al. similarly suggests increasing the nitrogen content of selected portions of a titanium nitride layer to yield dual metal gate electrodes in a Metal Insulating Semiconductor Field Effect Transistor (MISFET). Here, however, nitrogen ion implantation is proposed as a replacement for the ambient atmosphere exposure suggested by Liang et al. U.S. Pat. No. 6,815,285 similarly uses nitrogen ion implantation to selectively modify the work function of a metal layer.
U.S. Pat. No. 6,537,901 to Cha et al. takes a slightly different direction. Cha et al. form separate metal layers under separate processing conditions to yield dual metal gate electrodes having disparate PMOS and NMOS compatible work functions. Here again, however, differing amounts of nitrogen ultimately define the respective work functions of both metal layers.
Japanese patent publication 2004-111549 in the name of Hiroyuki uses selective metal ion (nickel) implantation into a deposited metal layer (tantalum) to effect similar results. However, metal doping of metal to produce changes in the doped metal layer's work function is expensive and sometimes yields inconsistent results. As a result, conventional approaches to metal layer work function adjustment are dominated by nitrogen doping in one form or the other.